- BACKGROUND OF THE INVENTION
This invention relates to a simple technique for the synthesis of magnetic nanocomposites. The technique is based on ion exchange and precipitation within a polymer matrix. The matrix could be in the form of powders, fibers, tubes and self-supported films or other three dimensional structures.
Prior attempts to prepare magnetic nanocomposites have utilized ground or milled particles of magnetic materials which were then dispersed in a carrier matrix, coated on to fabrics or added to finely ground, dispersed resins or zeolites. For example, Forder et al (“Preparation and Characterization of Superparamagnetic Conductive Polyester Textile Composites”, J. Mater. Chem, 3 (6) p 563-569 (1992)) describes the preparation of magnetic colloids which are then coated onto the surface of a polyester fabric. Zhang et al (“Generation of Magnetic Metal Particles in Zeolite by Borohydride Reduction at Ambient Temperature”, J. Mater. Chem, 6(6) p 999-1004 (1996)) treats sodium mordenite, a form of the naturally occurring zeolite designated hydrated calcium sodium potassium aluminum silicate, with a water soluble salt of a metal, M2+, where M is iron, cobalt or nickel, to replace Na+ on the resin with the metal ion. An aqueous suspension of the resin is then reacted with NaBH4 to reduce the metal ion to the metal M, which remains within the resin particles.
Ziolo et al (Ziolo, R. F., E. P. Giannelis, B. A. Weinstein, M. P. O'Horo, B. N. Ganguly, V. Mehrotra, M. W. Russell, and D. R. Huffinan, “Matrix mediated synthesis of Fe 2 O 3 : A new optically transparent magnetic material”, Science 257:219-23 (1992)), reported on the preparation of Fe2O3 nanoparticles in sulfonated polystyrene-type 50-100 micron beads of ion exchange resin. They then had to be molded into monolithic structures at temperatures which modify the properties and characteristics of the nanoparticles. Sourty et al, (“Ferrite-LoadED Membranes of Microfibrillar Bacterial Cellulose Prepared by in situ Precipitation”, E. Sourty, D. H. Ryan and R. H. Marchessault, Chem. Mater., 10(7), 1755-7(1998)) and Raymond et al (“In Situ Synthesis of Ferrites in Cellulosics” L. Raymond, J.-F. Revol, D. H. Ryan, R. H. Marchessault; Chem. Mater.; 6(2); 249-255 (1994)) describe the formation of ferrites in cellulosics. Suber et al (“Synthesis, and Structural and Morphological Characterization of Iron oxide-Ion-Exchange Resin and-Cellulose Nanocomposites”, Applied Organometallic Chemistry, 15, 414-420 (2001)) reports on further studies of such materials. Shahinpoor et al reports on the treatment of ion exchange resins, such as Nafion with platinum salts to deposit platinum on or in the matrix (“Ionic Polymer-Metal Composites: I. Fundamentals”, Smart Mater. Struct., 10, 819-833 (2001)
Several patents have subsequently issued to Ziolo directed to magnetic nanocomposite compositions and processes for preparing these materials (U.S. Pat. Nos. 4,474,866, U.S. Pat. No. 5,714,536 and U.S. Pat. No. 6,148,920). In particular, these patents are directed to magnetic nanocomposite compositions containing nanocrystalline Fe3O4 particles formed in and stabilized by an ion binding polymeric matrix. In particular, granules of ion exchange polymer resin are suspended in a liquid and are then loaded with iron ions. The Iron ions are then chemically converted to a magnetic oxide. For example, polystyrene-(SO3—)2Fe+2 resin is reacted with NaOH and H2O2 or N2H4 and NaOH to yield polystyrene-(SO3 −Na+)n plus gamma Fe2O3, the oxide being dispersed in the polymer matrix with particle sizes from about 0.0001 to about 0.1 microns in diameter. The end product is a very fine powder of the resin including the magnetic oxide for use as a toner for reprographic application.
Treatment of oxides with sodium borohydride has been used since the early '70s to produce the oxide of the metal and to form nanoparticles. However, they were not called “nanoparticles” at that time. (W. O. Freitag, T. A. Sharp, A. Baltz, and V. Suchodolski, J. Appl. Phys., 50, p7801-3 (1979), Composition of iron powders prepared by a borohydride process” and T. Uehori, A. Hosaka, Y. Tokuoka, and Y. Imaoka, IEEE Trans. Magn. 14, p852-4 (1978) “MagneticProperties of iron-cobalt alloy particles for magnetic recording media.”). The W. O. Freitag article points out that borohydride reduction methods have been well established, though not necessarily for nanoparticle alloys.
U.S. Pat. No. 6,107,233 to Harmer is directed to the formation of spherically shaped porous microcomposites of a perfluorinated ion-exchange resins with inorganic oxides dispersed there through starting from a mixture of a water miscible inorganic oxide and a water miscible ion-exchange resin. The mixture is then mixed with an organic liquid in which neither of the oxide or resin is soluble to create a dispersion of the water-miscible phase, in the form of spherical bubbles throughout the organic phase, followed by gelation of the water miscible components into spherical particles.
- SUMMARY OF THE INVENTION
In general, nanomaterials can be fabricated with magnetic, magnetostrictive, or magneto-optic functionality. Phosphorescent nanocomposites have also been synthesized using the same technique. The nanocomposites can provide improved materials for various applications such as—
- a. magnetics for power converters
- b. actuators for artificial muscles, valves, micro-mirrors and micropumps
- c. magneto-optical wave guides and switches
- d. magnetics for guiding micro-catheters and for drug delivery
- e. magnetodielectric materials for microwave and rf devices
- f. applications requiring functional conformable materials controlled displacement or positioning devices including macro and micro devices.
BRIEF DESCRIPTION OF THE DRAWINGS
Magnetic nanocomposites and methods of preparing those nanocomposites from precursor materials are described. Suitable precursor substrate materials include film, membranes, fibers or fabrics of ionomeric or cellulosic polymers. Metallic ions which can be deposited as nanocomposites in these substrates include iron, cobalt, nickel, magnesium, zinc.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
FIG. 1 shows a chemical reaction for synthesizing a nanocomposite within an ion exchange polymer matrix incorporating features of the invention.
FIG. 2 is a color photograph showing the effect of nanoscale particles present at increasing concentrations synthesized in a matrix using the synthesis procedure of FIG. 1.
FIG. 3 is a electronmicrograph of the 5% v. sample shown in FIG. 2.
FIG. 4 is the generalized chemical structure of a cellulosic starting material for use in a process incorporating features of the invention.
FIG. 5 is a graph showing the change in magnetic properties resulting from producing larger Fe2O3 nanoparticles in a matrix by repeating the deposition cycle at least 8 times.
FIG. 6 is the graph of FIG. 5 with each cycle normalized to the sixth cycle.
FIG. 7 is a graph showing the change in magnetic properties resulting from producing larger MnFe2O4 nanoparticles in a matrix by repeating the deposition cycle at least 8 times.
FIG. 8 is a graph showing the change in magnetic properties resulting from producing larger CoFe2O4 nanoparticles in a matrix by repeating the deposition cycle at least 8 times.
Various different wettable polymers can be used as starting materials. These polymer materials may be provided in a wide range of forms including, but not limited to films, pellets, powders, fibers, fabrics and coatings on supporting materials. For illustrative purposes, ionomers containing ion exchange groups and cellulosic fibers are discussed in detail.
While not intending that the process be limited by theory, FIG. 1 illustrates a preparative process believed to occur in the synthesis of nanocomposite materials starting from a polymer matrix of an ionomer containing ion-exchange groups such as widely used for water filtration and in fuel cells. It is typically sold under various trade names including Nafion®, Dowex®, etc. The matrix is based on sulfonated Teflon polymer chains with hydrogen ions balancing the charge. The nanoscale magnetic and magneto-optic materials are synthesized within the polymer matrix using ion exchange and chemical precipitation procedures at a temperature of about 60° C. The polymer matrix restricts the agglomeration of the nanoparticles that form and maintains the nano-particles to within confined areas of the host matrix. The concentration of nanoparticles within the matrix can be increased by repeating the ion exchange and precipitation procedures and thus provides a means for controlling the volume fraction of nanoparticles. This procedure also provides a means for synthesizing different materials within the same host matrix—e.g. dielectric and magnetic nanoparticles to yield magneto-dielectric materials. Addition of a few drops of hydrogen peroxide results in the direct precipitation of metal oxides. Metallic magnetic materials can also be synthesized using the ion-exchange method with subsequent reduction of ions to metallic state using sodium borohydride. Stretching the host matrix in the form of fibers or films gives rise to elongated areas within which the nanoparticles precipitate and thus also provides a means for controlling the nanoparticle shape. This may be necessary in some applications to increase magnetic permeability.
The sulfonated Teflon® polymer, Nafion® has the formula:
where a portion of that polymer chain is represented by:
When a matrix composed of sulfonated polymer is placed in an aqueous solution of a soluble metal salt, H+ ions on the polymer side chains are replaced, in an ion exchange reaction, by the metal ions, for example Fe2+, Ni2+, Co2+, Mn2+, from the solution. However, other metallic ions of a lesser or greater volume can also be used. The resultant polymer with bound metal ions can then be heated at temperatures from about 20° C. to about 100° C., preferably around 60° C.-80° C., with alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide. This results in the Na+ or K+ replacing the metal ion and the formation of metal oxides in the form of nano composites entrapped within the polymer matrix. Alternatively, rather than oxidizing the sulfonate polymer, it may be reduced, for example by using sodium borohydide, to form metallic alloys. While elevated temperatures result in an increased reaction rate, the reaction is may be further accelerated as a result of a substantially increased ionic mobility of the metal ions. Examples of suitable metal salts include, but are not limited to soluble salts of iron, cobalt, nickel, magnesium, vanadium, chromium and zinc. These soluble salts can be in the form of chlorides, iodides, bromides, fluorides, sulfates, acetates, nitrates, perchlorates thiocyanates, thiosulfates and the like. However, one skilled in the art will recognize the soluble salts with anions and cations may be used in the described procedure. Preferred salts provide Fe2+, Ni2+, Co2+, Mn2+, Fe3+, Zn2+. These salts may also be provided alone or in combination. When combined, mixtures of oxides or bimetallic oxides (or tri-metallic or multimetallic oxides) designated as MO can result. Typical nanocomposites produced include Fe2O3, MnFe2O4, CoFe2O4, NiFe2O4, Ni—Fe alloys and ZnO.
While FIG. 1 shows the starting polymer matrix to contain H+ ions, other soluble exchangeable ions may be used. For example, the Na+ containing molecule shown as the end product in FIG. 1 can be recycled and used as the starting material for exchange with the metal ion.
FIG. 2 is colored photographs showing nanoparticle of Mn/Fe2O4 formed with a polymer film according the above described procedure with the film containing 5% v, 15% v and 25% v of the nanoparticles. The film was colorless prior to treatment. The nanoparticles or nanocrystals are from 5 to 10 nm in size. The volume fraction of the nanocrystals is increased by repeating the ion exchange and precipitation procedures. The increase in nanoparticle concentration is readily apparent by the deepening of the red color of the polymer film with increased concentration.
FIG. 3 is an enlarged view of nanocrystals of 5%v MnFe2O4 at % v in a Nafion film.
Cellulosic materials such as cotton, linen, rayon and paper products may be used as starting materials. Alternatively, the technique includes oxidation of the hydroxyl groups to carboxylate groups followed by ion exchange and precipitation within a cellulosic structure. The structure could be in the form of pre-spun fibers, yarns, woven and non-woven textiles, wood, raw cotton, cotton bolls, paper or cardboard. The nanomaterials can be dielectric or magnetic. Magnetic functionality includes soft and hard magnets, or magnetoresistive, or magnetostrictive materials. Such nanocomposites are expected to provide improved materials for various additional applications such as—
- a. applications requiring functional conformable materials
- b. micro-magnetics for power generators in cloth forms (clothing, canvas covers, etc.)
- c. actuators for valves, and micropumps, electrical switches and micro-mirrors
- d. memory storage devices
- e. bar-coding (potentially invisible)
- f. proximity or direction sensors
- g. capillary peristaltic pumps
- h. controlled displacement or positioning devices including macro and micro devices.
FIG. 4 shows the generalized chemical structure of cellulosic materials, which are polymeric hydrocarbon non-aromatic six membered rings linked by oxygen atoms. Each ring contains a primary alcohol group —CH2 OH.
These primary alcohols can be converted to potassium carboxylate groups by oxidation with oxidizing agents such as potassium permanganate,
where R—CH2OH represents a repeating section of the cellulosic polymer.
The remainder of the process is similar to the procedure described above. The potassium ions are “ion exchanged” with other metal ions followed by precipitation and agglomeration. A soluble metal salt is ion-exchanged with the potassium in the carboxylated cellulose as follows:
nR—CO2 −K++Mn+→(R—CO2 −)nMn++nK+
where M can be Fe2+, Fe3+, Co2+, Ni2+, Mn2+, Zn2+, or a variety of other metal ions, or a combination of such metal ions, where the metals form insoluble oxides. After ion exchange, the metal ion is reacted with an alkali metal base such as sodium hydroxide in air,
n(R—CO2 −)Mn++NaOH/O2→n(R—CO2 −Na+)+M2On
to form the oxide. A 60° C. processing temperature is a preferred temperature. However, different temperatures in the range of from about 20° C. to about 100° C. may be more efficient due to the greater basisicity of the carboxylate group compared to the sulfonate group described above. This process can be repeated indefinitely to increase the size and/or concentration of nanoparticles within the cellulosic matrix. As with the process using the sulfonated polymer described above, reduction with sodium borohydrate yields metallic nanoparticles.
Unlike the sulfonated Teflon polymer, there are no physical bounds to particle growth in the cellulosic substrate. The nanoparticles will grow within the fiber or yarn structure. Size and concentration are controlled by reaction conditions including reactant concentration, reaction time and temperature, and the number of repetitions.
FIGS. 5 and 6 illustrates the change in magnetic properties for a series of films of Nafion containing increasing concentrations of iron oxide nanocomposite, from approximate 1%w 16%w, as a result of repeating the process described above multiple times, in this instance 1, 2, 4, 6 and 8 repetitions. The increase in concentrate was initially estimately to be approximate 2% repetition. FIG. 6 shows the other curves normalized to the curve for 6 repetitions. FIG. 7 compares the magnetic properties for Nafion—MnFe2O4 films with MnFe2O4 (4 repetitions) and MnFe2O4 (8 repetitions with the 8 repetition curve scaled 1.391 times). While the process was expected to procedure 2% for each repetition. In reality, it was found to produce 1.39% per repetition. In a like manner, FIG. 8 compares the magnetic properties for Nafion—CoFe2O4 films with CoFe2O4 (4 repetitions) and CoFe2O4 (8 repetitions), When the curves scaled 1.47 times the curve is substantially the same for the IV and VIII repetition CoFe2O4 film. These graphs demonstrate that there is no affection due to loading concentrations at heavy (IV and VIII repetitions) to account for actual loading vs. expected loading, on magnetic properties.
As indicated above, this procedure also provides a means for synthesizing different materials and combinations of materials within the same host matrix. Further, different metal oxides can be deposited in subsequent cycles of the process. For example, and solely for illustrative purposes, FeO2 can be deposited in early stages followed by NiF2O4 which may then be followed by CoFe2O4. In this manner, a flexible matrix with specifically designed magnetic properties or functions can be constructed.
Other metallic magnetic materials as well as other unique compositions or alloys can then be produced by further chemical processing. As an example, by reacting the oxide composite with sodium borohydride (NaBH4) magnetic materials such as permalloy (NiFe), samarium-cobalt, or neodymium-boron can be formed. The borohydride can also reduce the carboxylate and hydroxyl groups in the cellulosic structure resulting in various different cellulosic reaction products, resulting from, but not limited to, cleavage of the carboxyl group, crosslinking of the polymer or combinations thereof.
In a typical synthesis starting with a substrate of a Nafion TMC proton exchange membrane (5 to 10 cm (2″ to 4″) square with a thickness of 50 micrometers (0.002″)) or perfluorosulfonic acid polymer beads or fiber (30-40 g of substrate), the following procedure was used.
1. The substrate material is ion exchanged at room temperature with stirring for 1 hour. typically in 400 ml aqueous solution. Examples of the solutions used for specific nanomaterials are:
|Solution Composition ||MetalOxide Produce |
|1:1 ratio of 0.05 M MnCl2, and 0.1 M FeCl2 ||MnFe2O4 |
|1:2 ratio of 0.05 M Co(NO3)2 and 0.1 M FeCl2 ||CoFe2O4 |
|0.1 M FeCl2 ||Fe2O3 |
|0.0358 M FeCl2 and 0.1368 M NiSO4 ||NiFe (80% Ni) alloy |
|0.1 M Zn (NO3)2 - (note: ZnO is non-magnetic; ||ZnO |
|however, it is phosphorescent) |
- 2. After ion exchange the substrate is washed thoroughly using deionized water.
- 3. To produce CoFe2O4, Fe2O3, or ZnO
20 ml of 12 M NaOH with 2 ml 10% H2O2 (hydrogen peroxide) is then added dropwise into the ion-exchanged material at 80° C. The H2O2 ensures that the Fe (II) is oxidized to Fe (III). The substrate is then heated for about 30 minutes to complete the reaction.
The substrate is then washed thoroughly using deionized water preferably 7-10 times. The steps above are repeated to increase nanoparticle loading. For example, 20 repetitions results in 30% to 45% by volume loading of the nanomaterial. The percentage by weight is much higher since the density of the nanomaterials is greater than the polymer.
- 4. For MnFe2O4 or NiFe 12M NaOH is used without the hydrogen peroxide.
For NiFe alloy, after reaching the desired loading the product is reduced using 30 ml of 2.5 M NaBH4 (sodium borohydride)
Using a 2″-3″ square of a prewashed textile composed of cellulosic fibers (a fine cotton weave, 90 picks/inch) as a substrate, the substrate material is prepared by oxidizing the —OH (hydroxyl) groups in cellulose to —COOH (carboxylic acid groups).
This is accomplished in two steps. First, the cellulose is oxidized with aqueous KMnO4 (typically 0.05 M). This yields the carboxylate salt RCOO−K+ where R is the cellulosic backbone
The carboxylate is then converted to the acid form
The procedure used for the Nafion or perfluorosulfonic acid polymer described above is then used to precipitate ion-oxide particles within the fibers of the fabric. Ion exchange typically requires 12 hrs for completion rather than 1 hour as above. Two reasons for this difference are that the carboxylic acid proton is less mobile than the protons in highly cationic ion-exchange polymers, and the capacity for ion exchange in the cellulosic material is much lower.
While the process above is described for specific soluble metallic salts, one skilled in the art will recognized that the process may be used for a broad range of soluble metallic salts which may be converted to insoluble forms (i.e. insoluble oxides) by subsequent reactions. Also, one skilled in the art will recognized that the described process is not limited to the production of matrix materials with magnetic properties. A broad range of metals or metal containing precipitates can be formed within the matrix for numerous other applications, including, but not limited to phosphorescent materials, energy absorbing materials (i.e. electromagnetic, nuclear radiation), semiconductors or high strength composites or precursors for those composites.
One skilled in the art will also recognize that the precursor materials are not limited to the ionomers or cellulosic materials described. Other polymeric materials with reactive pendant replaceable groups (i.e., H+ Na+, K+ etc.) —OH, COOH, or groups replaceable with other pendent groups which will react with or exchange with cautions in the metal salt may be used. Examples of other useable polymer substrates include polyamides, epoxies, polyurethanes, vinyl, phenolics and polyester resins.
The processes described above result in magnetic or magnetodielectric nanocomposites processed directly into final shapes such as fibers, films, tubes and textile sheet. No machining, tape casting and other processes are required. This provides the ability to directly make an nanocomposite in a final desired shape in one processing step. While powders of nanocomposites made by ion-exchange and precipitation are shown in the literature they are not suitable for use in the formation of finished shapes since further processing to form these shapes requires mixing and dilution with a binder or other polymer and heating to make a solid body. This causes nanoparticles to grow and the superior magnetic properties will cease to exist.
The shape, size and volume fraction of the magnetic and magnetodielectric nanoparticle embedded in the polymer matrix has a profound effect on its final properties. The process described herein allows for control of the nanoparticle shape, size and volume fraction. The nanocomposite powders (or beads) made by prior art techniques only yield spherical particles because the shape of the ionic cluster is spherical. The spherical shape is not desirable for increasing the magnetic permeability or dielectric constant of a nanocomposites. In fibers, films and tubes the ionic cluster is ellipsoidal due to stretching and this yields ellipsoidal nanoparticles, with the permeability higher in one direction. The interaction between the nanoparticles is determined by the spacing between them and their volume fraction, both of which can be increased in the process described by repeating the ion exchange and precipitation procedures. This also has a profound effect on permeability, permittivity, saturation flux density and loss. The process described provides the ability to synthesize magnetic nanoparticles in a magnetic field or ferroelectric nanoparticles in an electric field, thus imparting anisotropy, which is desirable for certain applications.
Mixed compounds (such as MnFe2O4) and alloys (such as Ni—Fe) can be made by this process due to the close proximity of ion-exchange sites in the polymer. As an example, the process can provide mixed compounds rather than separate MnO and Fe2O3 nanoparticles or separate Ni and Fe nanoparticles. Mixed compounds form spinals and therefore have much higher magnetization (or dielectric constant in case of ferroelectric nanoparticles) than individual compounds. Unique alloys, for example such as samarium-cobalt or neodymium-iron-boron can be produced by using solutions of mixed soluble salts are sequentially applying different salt solutions. By utilizing excess borohydride some boron may also be incorporated in the end product.
Also, mixtures of either mixed or simple compounds can be made. This is a unique feature of the process described herein which is not shown in the prior art. In other words, one can make compound A in a first procedure and then make compound B adjacent to compound A in a second step. As an example, nanocomposite magnetodielectrics can be synthesized. The presence of an electric field in a ferroelectric nanoparticle adjacent to ferromagnetic or ferrimagnetic nanoparticle can accentuate the permeability and permittivity in ways that the individual compounds do not exhibit. In a like manner red, green and blue nanophosphors can be synthesized in the same polymer film rather than requiring three separate films to yield white light.
These nanocomposite films also have conformability and flexibility. No prior art process has been demonstrated which is capable of producing high performance magnetic and magnetodielectric materials that are flexible. Prior available pure ceramics or metals and composites are either brittle or exhibit large losses at high frequency and pure polymers do not exhibit large enough permittivity or permeability. Composites made by prior art techniques of mixing polymers with ceramic or metallic functional materials are restricted to low volume fractions (10-20%) which is not high enough to make high performance conformable materials. The processes described can produce conformable nanocomposites with suitable properties having 40-50% v metallic or metal oxide functional nano-materials. These nanocomposite films also preserve the mechanical strength and thermal stability of the matrix material because of reinforcement provided by the nanocrystallites in the composite so formed.
It has been also found that the nanocomposite films produced by processes incorporating features of the invention swell or expand as nanoparticles are deposited therein. This allows for further loading of additional functional nanoparticles without agglomeration. This allows the synthesis of magnetic and ferroelectric nanoparticles in close proximity to each other, imparting certain properties not found in prior produced individual phases.
The nanocomposite fibers made by the process set forth herein can be embedded into cotton textiles or the cotton textile itself can be rendered magnetic or magnetodielectric. As a result, the process has the ability for forming ferroelectric and ferromagnetic (or ferrimagnetic) nanoparticles in cotton to provide electronic textiles usable for antennas, power converters, EMI suppression, etc. Using Zn+2 salts (for example Nitrate or chloride salts) as starting materials, the procedures described herein also allow direct synthesis of monolithic films of Nafion or cellulosic with concentrations of up to about 20%w of phosphorescent nanoparticules. These films have been demonstrated to emit visible light when exposed to UV light. Phosphorescent nanoparticles deposited by the process described possess higher luminescent efficiency then the same materials in bulk. Further, high refractive index nanoparticles can be synthesized in the film along with the phosphor nanoparticles, providing higher efficiency light coupling. Still further, multiple different colored phosphors, such as red, green and blue phosphors can be synthesized within the same film, thus allowing white light emission from a monochromatic light source.
While the invention is primarily directed to the formation of magnetic materials, the teachings herein are applicable to a broad range of metals or combinations of metals which form soluble salts, for example Ga, AgAs, Se, InSb and Si. These metals can be attached to the polymer in a like manner using a borohydride or more active reducing agents as a reducing agent. It is also not necessary to limit the process to aqueous solutions; other solvents can be used as long as they do not negatively affect the substrate material. Still further, using the described process it is possible to produce elongated particles by subjecting the polymer substrate to tension, or drawing the polymer into elongated fibers during the formation process.